Deformation mechanisms under shallow foundations on liquefiable layers of varying thickness

Structures with shallow foundations resting on liquefiable layers can suffer excessive settlement in the event of an earthquake. The state of practice often estimates the settlement of structures using empirical methodologies. Commonly, these are based on case histories or estimations developed for the free field. Their reliability has been contested due to uncertainties regarding the dominant deformation mechanisms in the presence of a structure. Here, six dynamic centrifuge tests are presented, investigating the response of structures with shallow foundations resting on liquefiable layers of different thickness. Particle image velocimetry (GeoPIV) was used to capture the developed deformation mechanisms. A structure resting on a deep liquefiable layer was found to settle primarily due to increased lateral soil displacements taking place beneath a bulb of stiffer soil formed below the foundation. In shallower layers, this bulb reached the base of the layer, transmitting large accelerations to the structure and promoting a rocking response. Settlement in this case was generated due to increased soil displacement from under the edges of the foundation. In no case were methodologies aimed for the free field able to account for the salient settlement-generation mechanisms.